Acousto-optic beam steering system

ABSTRACT

Systems and methods for steering an optical beam in two dimensions are disclosed. The system includes a substrate comprising an acousto-optic antenna array and an acoustic transducer. Each antenna of the antenna array includes a high-confinement surface waveguide carrying a light signal. The acoustic transducer imparts acoustic energy into each surface waveguide as a mechanical wave. Interaction of the light signal and mechanical wave in each surface waveguide induces light to scatter into free space. The light scattered out of the plurality of waveguides collectively defines the output beam. The longitudinal angle of output beam, relative to the substrate, is determined by the relative frequencies of the mechanical waves and the light signals. The transverse angle of the output beam is controlled by controlling the relative phases of the mechanical waves and/or light signals across the surface-waveguide array.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/527,332 filed Jun. 30, 2017. The entire disclosure of U.S.Provisional Application No. 62/527,332 is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under contract 1509107awarded by the National Science Foundation and under contractN00014-15-1-2761 awarded by the Office of Naval Research. The Governmenthas certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to optical beam steering in general, and,more particularly, to optical-phased-array-based beam steering.

BACKGROUND

Two-dimensional optical beam steering is an important technology thathas found widespread use in a variety of applications, includingtelecommunications, LiDAR, three-dimensional imaging, hyperspectralimaging, and optical sensing applications, among others.

Historically, most beam-steering systems employed slow and expensivemacro-mechanical-beam-deflection systems such as two-dimensionalgimbal-based movable mirrors, pairs of one-dimensional movable mirrors,position-controllable bulk optics, and the like. Unfortunately, suchsystems are typically complex to implement, are quite costly, havelimited response speeds, and are fraught with reliability issues.

In contrast, on-chip optical phased arrays (OPAs) offer a substantiallysolid-state approach to optical beam steering that promise faster, morerobust, and less expensive beam-steering systems. Prior-art OPAs havebeen demonstrated using widely disparate technologies, such asliquid-crystal phase shifters, MEMS-based piston or grating-basedmirrors, and integrated-optics-based surface waveguide arrays havingintegrated grating couplers and thermo-optic phase shifters.Unfortunately, while such prior-art systems offer lower cost and higherreliability than macro-mechanical beam-deflection systems, they stillhave significant drawbacks.

Liquid-crystal-based phase shifters, for example, are known to haverelatively slow response times and are highly sensitive to changes intemperature and wavelength.

Micromechanical (i.e., MEMS-based) OPAs employing piston-actuatedmirrors can achieve fast response times; however, it can be difficult torealize a MEMS-based OPA having small inter-element pitch, which limitsthe size of the field-of-view (FOV) that can be achieved.

OPAs based on integrated-optics-based surface waveguide gratings andtunable lasers also show promise. In such systems, rapidly tuned lasersare used to change the direction of the light by exploiting the highdispersion of the surface waveguide gratings. Unfortunately, thisrequires implementing rapidly tunable lasers in silicon photonics, givesrise to cross-talk between transmit and receive on the same antenna, andlimits the RF bandwidth that can be transmitted on the light field dueto angular dispersion.

Furthermore, the geometric perturbations that define the surfacewaveguide gratings suitable for OPA use are preferably defined usingstandard lithography techniques to define of the gratings. To realize anoutput light beam with low beam divergence, however, long waveguides(several millimeters or more) and high-resolution patterning (on theorder of one nanometer) can be required. Unfortunately, patterningnanometer-scale features over large areas can be difficult, if notimpossible, using standard patterning techniques. As a result, high-costpatterning methods, such as e-beam lithography, x-ray lithography, andthe like, are required. Alternatively, the surface waveguides and thegratings therein must be enlarged to increase the required lithographydimensions, thereby reducing waveguide packing density.

Fixed-frequency lasers have been employed to mitigate thesedisadvantages; however, this leads to other factors arise that can makeon-chip two-dimensional (2D) beam steering extremely challenging. Forexample, the number of phase shifters and couplers required scalesunfavorably with the area of the aperture. In addition, the most commonphase shifter is a thermo-optic phase shifter, which has poor powerefficiency. As a result, milliwatts of static power consumption istypically required per element. For a millimeter-scale aperture,therefore, 2D beam steering consumes kilowatts of power just forpowering the phase shifters themselves.

Still further, unlike phased arrays at microwave frequencies wheremicrostrip patch antennas can radiate efficiently and have footprintssmaller than the free space wavelength of the electromagnetic field, inthe optical domain, it is difficult to fully scatter light from anoptical guided wave to a radiating mode with a small footprint (sub-freespace-wavelength). As a result, the radiating elements or antennae in aphotonic system are typically large and spaced in two dimensions with apitch greater than the wavelength of the light. The large spacing leadsto significant side-lobes and a reduction in phased array performance.

The need for a device technology that enables practical optical beamsteering in two dimensions remains, as yet, unmet in the prior art.

SUMMARY

The present disclosure enables optical beam steering by employingdynamic grating structures in each of an array of surface waveguides.The dynamic gratings are formed by imparting mechanical waves having adesired frequency in the surface waveguides. As a result, in eachsurface waveguide, the optical wave propagating through it is subject toan acousto-optical interaction that causes the optical wave to scatterout of the surface waveguide as free-space optical radiation. The lightscattered from the array of surface waveguides collectively defines oneor more optical beams that can be independently steered in a firstdimension (the longitudinal dimension) by controlling the wavelength ofthe mechanical waves. Steering in a second dimension (the transversedimension) is performed by controlling the relative phases of themechanical and optical waves in each surface waveguide. Furthermore, byemploying materials compatible with standard silicon photonicsprocesses, embodiments in accordance with the present disclosure arecompatible with previously developed technologies in silicon photonics(e.g., lasers, phase-shifters, detectors, etc.) and enable monolithicintegration of electronic circuitry, thereby affording system-scaleintegration. Embodiments in accordance with the present disclosure areparticularly well suited for use in applications such as light detectionand ranging (LIDAR), autonomous vehicles, robotics, light-field imaging,plenoptic cameras, free-space optical communications and switchingsystems, wireless power transfer, medical diagnostics, and the like.

In contrast to the prior art, embodiments in accordance with the presentdisclosure employ one or more high-confinement waveguides, which affordssignificant advantages over prior-art acousto-optic systemsincluding: 1) more efficient light scattering; and 2) higher waveguidepacking density, which enables better suppression of undesirablesidelobes in the output signals of the beam steering systems.

An illustrative embodiment is an optical beam-steering system comprisingan array of high-confinement waveguides, each of which is operativelycoupled with a mechanical transducer that imparts a mechanical wave inthe material of the surface waveguide. The mechanical transducer iscoupled with each surface waveguide through a mechanical phase shifterthat enables control over the phase of the mechanical wave in itsrespective surface waveguide. Two-dimensional beam steering is enabledby controlling the wavelength of the mechanical waves in the surfacewaveguides, as well as the relative phase of the mechanical waves acrossthe surface waveguide array.

In some embodiments, a high-index-contrast slab waveguide is usedinstead of an array of individually defined waveguides. Optical and/ormechanical phase shifters control the direction of light and/ormechanical waves in the slab thereby directing outgoing radiation.

In some embodiments, the surface waveguides are held above the substrateby narrow support structures such that most of the structural materialof the surface waveguides is movable relative to the substrate.

In some embodiments, each surface waveguide includes a phase shifterthat controls the phase of its respective light signal. In suchembodiments, the inclusion of mechanical phase shifters is optional.

In some embodiments, at least one of the surface waveguides is opticallycoupled with a photonic lightwave circuit that includes one or morephotonic elements such as lasers, phase shifters, detectors, gainelements, modulators, diffraction elements, Bragg mirrors, splitters,combiners, and the like. In some embodiments, the surface waveguidearray is part of a photonic integrated circuit that also includeselectronic circuitry, logic elements, and the like.

An embodiment in accordance with the present disclosure is an opticalbeam steering system comprising: a substrate; a plurality of surfacewaveguides disposed on the substrate, each surface waveguide conveying alight signal, and each surface waveguide being a high-confinementwaveguide; an acoustic transducer that is operatively coupled with theplurality of surface waveguides, the acoustic transducer beingconfigured to induce a plurality of mechanical waves such that eachmechanical wave of the plurality thereof is coupled with a differentsurface waveguide of the plurality thereof; and a phase controller forcontrolling the relative phase of each mechanical wave of the pluralitythereof and the light signal conveyed by its respective surfacewaveguide.

Another embodiment in accordance with the present disclosure is a methodfor steering an optical beam, the method comprising: conveying aplurality of light signals having wavelength λ in a plurality of surfacewaveguides disposed on a substrate, each surface waveguide conveying adifferent light signal of the plurality thereof, and each surfacewaveguide being a high-confinement waveguide; coupling a plurality ofmechanical waves into the plurality of surface waveguides such thatinteraction between the plurality of mechanical waves and the pluralityof light signals gives rise to emission of an optical beam from theplurality of surface waveguides, wherein the mechanical waves arecharacterized by first frequency, ω; and controlling the firstfrequency, ω.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a functional block diagram of a beam-steering system inaccordance with an illustrative embodiment in accordance with thepresent disclosure.

FIG. 2A demonstrates the generation of free-space radiation viainteraction between a light signal propagating in a single surfacewaveguide and a mechanical wave.

FIG. 2B depicts the relationship between phase-matching between theguided optical and mechanical waves and the coupling angle, θ.

FIG. 3A depicts a schematic diagram of a detailed perspective view ofsystem 100.

FIG. 3B depicts a schematic drawing of a cross-sectional view of aportion of acousto-optic antenna array 102.

FIG. 4 depicts operations of a method for forming a beam-steering systemin accordance with the illustrative embodiment.

FIG. 5 depicts a schematic of a cross-section view of acousto-opticantenna array 102.

FIG. 6 depicts operations of a method for steering an optical beam intwo dimensions in accordance with the illustrative embodiment.

FIG. 7 depicts a beam-steering system in accordance with an alternativeembodiment in accordance with the present disclosure.

FIG. 8 depicts a beam-steering system in accordance with anotheralternative embodiment in accordance with the present disclosure.

DETAILED DESCRIPTION

FIG. 1 depicts a functional block diagram of a beam-steering system inaccordance with an illustrative embodiment in accordance with thepresent disclosure. System 100 comprises acousto-optic antenna array102, light source 104, acoustic source 106, phase controller 108, andprocessing circuit 110. System 100 is operative for forming and steeringoutput beam 116 in two dimensions.

Acousto-optic antenna array 102 is a planar lightwave circuit (PLC)comprising a plurality of high-index-contrast, high-confinement surfacewaveguides formed on a substrate, where each surface waveguide functionsas a different antenna element of the antenna array, and where the PLCis configured to efficiently couple mechanical energy provided byacoustic source 106 into the surface waveguides.

As will be understood by one skilled in the art, anintegrated-optics-based optical waveguide (referred to herein as a“surface waveguide” or “waveguide”) is a light-guiding structure formedon the surface of a substrate. The light-guiding structure comprises alight-guiding core that is surrounded by cladding layers that serve toconfine optical energy within the surface waveguide. The materials ofthe core and cladding are selected such that the refractive index of thecore material is at least slightly higher than the refractive index ofthe cladding material(s). The difference in these refractive indicesdictates how tightly the optical energy of a light signal propagatingthrough the surface waveguide is confined to the core region.

Typically, prior-art acousto-optic beam-steering systems are based onsurface waveguides whose core and cladding materials have only a slightdifference in refractive index resulting in low-confinement of thelight. Typically, prior-art acousto-optic beam-steering systems arebased on surface waveguides that are not released from their substrateresulting in low-confinement of the mechanical waves. These surfacewaveguides are referred to, herein, as “low-confinement surfacewaveguides.” In contrast, embodiments in accordance with the presentdisclosure employ high-confinement waveguides, which affords themsignificant advantages over the prior art, as discussed below. For thepurposes of this Specification, including the appended claims, a“high-confinement waveguide” is defined as an integrated-optics-basedoptical waveguide whose light-guiding core comprises a material having arefractive index that is at least 10% higher than the refractive indexof the cladding material that surrounds the core and is released fromits substrate rendering it “mechanically active.” As a result, theoptical or mechanical energy of a light or acoustic signal propagatingthrough a high-confinement waveguide has a mode field that extends onlyslightly (if at all) into the cladding regions of the surface waveguide.In the depicted example, the high-confinement waveguides employed insystem 100 are air-cladded, silicon-core surface waveguides has a corecomprising silicon, which is surrounded by air, where the air functionsas the cladding material of the surface waveguide. Silicon has arefractive index of approximately 3.5, while air has a refractive indexof 1. Other examples of high-confinement waveguides suitable for use inembodiments in accordance with the present disclosure include, withoutlimitation, surface waveguides having cores of silicon nitride andcladdings of air and silicon dioxide, silicon-core surface waveguideshaving at least one silicon dioxide-based cladding layer, certainsilicon oxynitride-core waveguides, and the like.

Light source 104 is a laser that provides light signals 112, each ofwhich has a wavelength, λ1, of approximately 1.55 microns and isoptically coupled into the surface waveguides of acousto-optic antennaarray 102. In some embodiments, light source 104 is a different sourceof coherent light. In some embodiments, light signals have a wavelengthother than 1.55 microns.

Acoustic source 106 is a generator that is operative for generatingacoustic energy 114 that couples into each antenna (i.e.,high-confinement waveguide) of acousto-optic antenna array 102 in theform of a mechanical wave having frequency, ω.

Phase controller 108 is a controller operative for controlling thephases of each of the mechanical waves in the surface waveguides ofacousto-optic antenna array 102. In some embodiments, phase controller108 includes a plurality of thermal tuning elements operative forcontrolling the elasticity and/or density of material between acousticsource 106 and each surface waveguide. In some embodiments, phasecontroller includes a plurality of elements that controls the phase ofthe mechanical waves in each surface waveguide via another physicalmechanism. In some embodiments, phase controller 108 includes an arrayof optical phase controllers that control the optical phase of each ofthe light signals propagating through the surface waveguides ofacousto-optic antenna array 102.

Processing circuit 110 is an electronics circuit that includes aconventional processor operative for, among other things, controllingthe output of light source 104, acoustic source 106, and phasecontroller 108 to steer output beam 116 in two dimensions. In thedepicted example, processing circuit 110 is monolithically integrated onthe same substrate as acousto-optic antenna array 102. In the depictedexample, the processing circuit is depicted as a single, discretecomponent. In various other embodiments, the processing circuit can bedistributed, at least in part, among multiple components of system 100,implemented, in part or in full, in a remote or cloud-based computingsystem, or otherwise implemented in a suitable arrangement for carryingout the functions described herein.

Operating Principle of the Optomechanical Antenna

The operative principle of embodiments in accordance with the presentdisclosure arises from the fact that an acoustic wave coupled into asurface waveguide can efficiently scatter light propagating as a guidedoptical wave in the waveguide into a beam that propagates in free space.This is because phase-matching between two optical waves propagating inthe longitudinal direction can be achieved using the acoustic wavemomentum, while out-of-plane momentum does not need to be conserved foroptical waves at a surface that breaks translational symmetry in thevertical direction. As discussed below, the longitudinal angle at whichthe output beam propagates depends on the relative wavelengths of theacoustic wave and guided optical wave.

FIG. 2A demonstrates the generation of free-space radiation viainteraction between a light signal propagating in a single surfacewaveguide and a mechanical wave. Plot 200 shows guided optical wave 202as injected into surface waveguide 204 such that it is characterized byfrequency, ω, within the surface waveguide.

Mechanical wave 206 is induced in surface waveguide 204 such that themechanical wave and guided optical wave 202 interact over an interactionlength, which gives rise to scattering of the guided optical energy intofree space. The mechanical wave induces changes in the opticalproperties of the surface waveguide materials by, for example,increasing the path length, inducing an index change due to a volumeelasto-optic effect, and/or realizing a boundary perturbation effect.

Because energy and momentum conservation requirements must be obeyed,interaction between the mechanical wave and guided optical wave giverise to free-space radiation 208, which is Doppler shifted such that itis characterized by frequency ω_(rad), where ω_(rad)=ω+Ω and Ω is thefrequency of mechanical wave 206.

In the ideal case, waveguide momentum conservation yields the phasematching condition:β(ω)−K(Ω)=k ₀(ω_(rad))cos(θ),  (1)where β(Ω) is the wavevector of guided optical wave 202, K(Ω) is thewavevector of mechanical wave 206, θ is the coupling angle (i.e., anglein the x-z plane relative to the surface of the waveguide) at which theoutput radiation propagates, and k₀=ω_(rad)/c (where c is the speed oflight).

Therefore, by controlling the phase relationship between mechanical wave206 and guided optical wave 202, the magnitude of coupling angle θ canbe controlled.

FIG. 2B depicts the relationship between phase-matching between theguided optical and mechanical waves and the coupling angle, θ. Plot 210depicts coupling angle, θ, for three different phase-matching conditionsin the system shown in plot 200.

It should be noted that the induced index changes due to the volumeelasto-optic effect, as well as the boundary perturbation effect aresignificantly enhanced in embodiments in accordance with the presentdisclosure due to the fact that the surface waveguides included inacousto-optic antenna array 102 are high-confinement waveguides ratherthan low-confinement waveguides as used in prior-art systems. As aresult, embodiments in accordance with the present disclosure requirelower amplitude mechanical motion to effect a desired amount of lightscatter out of the surface waveguides.

Returning now to the illustrative embodiment, FIG. 3A depicts aschematic diagram of a detailed perspective view of system 100. As shownin FIG. 3A, system 100 is an example of a beam-steering system in whichacousto-optic antenna array 102, acoustic source 106, and phasecontroller 108 are monolithically integrated on substrate 302. In thedepicted example, system 100 also includes monolithically integratedprocessing circuit 110. In some embodiments, at least one ofacousto-optic antenna array 102, acoustic source 106, phase controller108, and processing circuit 110 is formed on a separate substrate andintegrated in a hybrid manner (or otherwise operatively coupled) withthe remaining elements of system 100. In some embodiments, light source104 is monolithically integrated on substrate 302.

FIG. 4 depicts operations of a method for forming a beam-steering systemin accordance with the illustrative embodiment. Method 400 begins withoperation 401, wherein the structure of the surface waveguide elementsof acousto-optic antenna array 102 is defined on substrate 302. Method400 is described with continuing reference to FIGS. 1 and 3A-B.

Substrate 302 is a conventional substrate suitable for use in a planarprocessing fabrication method. In the depicted example, substrate 302 isa conventional silicon-on-insulator (SOI) substrate comprisingconventional silicon handle wafer 316, buried oxide layer (BOX) 318, andsingle-crystal-silicon active layer 320, where the thickness of activelayer 320 is approximately 300 nanometers (nm).

It should be noted that, while substrate 302 is an SOI substrate in thedepicted example, myriad alternative substrates can be used forsubstrate 302 without departing from the scope of the presentdisclosure. Substrates suitable for use in embodiments in accordancewith the present disclosure include, without limitation, substratescomprising silicon, lithium niobate, compound semiconductors (e.g.,gallium arsenide, aluminum gallium arsenide, indium phosphide, cadmiumtelluride, etc.), semiconductor compounds (e.g., silicon carbide,silicon germanium, etc.), dielectrics, dielectric stacks, glasses,composite materials, and the like.

Acousto-optic antenna array 102 includes a linear array of surfacewaveguides 304. Each of surface waveguides 304 includes at least aportion that is a straight waveguide, and these straight-waveguideportions are arranged in parallel along the y-direction. For exemplarypurposes, acousto-optic antenna array 102 is depicted in FIG. 3 ashaving only five antenna elements (i.e., surface waveguides 304). Itshould be noted that, typically, the number of antennae included inacousto-optic antenna array 102 is within the range of 10 to 10,000;however, acousto-optic antenna arrays in accordance with the presentdisclosure can include any practical number of antennae.

FIG. 3B depicts a schematic drawing of a cross-sectional view of aportion of acousto-optic antenna array 102. The view depicted in FIG. 3Bis taken through line a-a as shown in FIG. 3A. Each of surfacewaveguides 304 is an air-cladded, silicon-core ridge-type surfacewaveguide that is held above handle wafer 316 by distance d1 withinmechanically active region 314.

The structure of surface waveguides 304 is formed in mechanically activeregion 314 by defining the lateral extent of the ridge portion of eachwaveguide structure in a photoresist mask disposed on active layer 320and partially etching the exposed regions of the active layer in aconventional reactive-ion etch (RIE).

In the depicted example, each of surface waveguides 304 is a ridge-typewaveguide comprising ridge portion 328 and slab region 330. Ridgeportion 328 has width w1 and thickness t1 and slab region 330 has widthw2 and thickness t2. The ridge portions project from a substantiallycontinuous slab region with uniform pitch, p1 along the y-direction. Inthe depicted example, w1 and w2 are 500 nm, t1 is 300 nm, t2 isapproximately 100 nm, and waveguide pitch, p1 is 1500 nm. Typically,surface waveguides 304 are configured to guide the optical waves oflight signals 112 as well as the mechanical waves of acoustic energy114.

It should be noted that the dimensions provided above are merelyexemplary and that a wide range of waveguide dimensions can be usedwithout departing from the scope of the present disclosure. Furthermore,surface waveguides 304 can have any practical waveguide structure, suchas a channel waveguide structure, etc., and/or include any practicalcore and cladding materials (e.g., silicon nitride, silicon-rich siliconnitride, silicon dioxide, lithium niobate, compound semiconductors,etc.) Furthermore, as discussed below, in some embodiments, thesurface-waveguide array of acousto-optic antenna array 102 is replacedby a slab waveguide.

Preferably, surface waveguides 304 are formed in a “mechanically active”region of substrate 302. As a result, at optional operation 402,mechanically active region 314 is formed. In some embodiments, surfacewaveguides 304 are not disposed on a mechanically active layer.

Mechanically active region 314 is formed by removing BOX layer 318 fromunderneath it, thereby “releasing” that portion of the active layer fromhandle wafer 316. To form mechanically active region 314, release holes324 are etched through slab regions 330 to underlying BOX layer 318. Arelease etchant (e.g., hydrofluoric acid) is used to attack the BOXlayer through release holes 324, thereby undercutting the active layer320 and defining mechanically active region 314. This release etch istimed such that a portion of BOX layer 318 remains in place to definesupports 322, which inhibit lateral propagation of acoustic energybetween adjacent surface waveguides 304.

In some embodiments, mechanically active region 314 is supported abovehandle wafer 316 via a plurality of anchors, which are formed ofstructural material (e.g., polysilicon, silicon nitride, etc.)conformally deposited into vias formed through structural layer 320 andBOX layer 318 before the release etch is performed.

The inclusion of mechanically active region 314 mitigates coupling ofacoustic energy 114 into the handle wafer, thereby enabling a highlyefficient scatter process (as much as three orders of magnitude moreefficient than prior-art systems). As a result, the amount of powerrequired for device operation to effect light emission from the surfacewaveguides is reduced. It should be noted that a more efficient scatterprocess also reduces the required interaction length (i.e., the lengthover which mechanical waves 326 and light signals 112 interact).However, it is typically preferable for the interaction length to remainlong to effect good far-field resolution and reduce power consumption.One skilled in the art will recognize, after reading this Specification,that forming mechanically active region 314 gives rise to a mechanicaleffect that is somewhat analogous to the total internal reflection of alight signal within a surface waveguide and reduces the mechanicaldamping of the mechanical energy by the substrate, each of which canincrease the distance over which acoustic energy 114 can propagate onchip.

In the depicted example, the slab regions of the surface waveguides aremechanically connected along the entire length of the surfacewaveguides; however, in some embodiments, the slab regions are patternedto define discrete tethers that are distributed along the length of eachsurface waveguide such that that they extend between adjacentwaveguides. As a result, the tethers support the surface waveguidesabove handle wafer 316 and also mitigate mechanical coupling betweenadjacent surface waveguides. In addition, mechanical and/or opticalcross-talk between the surface waveguides can be inhibited by patterningthe slab in more complex ways (e.g. to make subwavelength surfacewaveguides), as well as by varying waveguide core dimensions to inhibitcoupling of optical and/or mechanical waves between distinct surfacewaveguides. Furthermore, surface waveguides 304 can be formed in anypractical material system, including, without limitation, silicon onoxide (suspended or unsuspended), silicon nitride, diamond, siliconcarbide, gallium nitride, gallium arsenide and its compounds, indiumphosphide and its compounds, aluminum nitride, lithium niobate, lithiumtantalite, and the like.

It is well understood that phased antenna arrays exhibit sidelobes whosesize depends strongly on the spacing between their antennae elements. Ifthe antennae are spaced by more than the wavelength of the light signalsbeing operated on, the sidelobes become comparable to the main beam.Furthermore, not only do the sidelobes become large, the angular densityof these lobes increases.

Because a low-confinement surface waveguide has a propagating opticalmode that extends well into its cladding, adjacent low-confinementsurface waveguides must be spaced apart by large distances to avoidcross-coupling of their optical modes. An acousto-optic antenna arraybased on low-confinement waveguides, therefore, would exhibit largesidelobes, reducing the optical power in the main output beam and thefield of view of the system.

As noted above, however, embodiments in accordance with the presentdisclosure employ acousto-optic antenna arrays based on high-confinementwaveguides. Since their optical modes do not extend significantly intotheir claddings, high-confinement waveguides can be spaced apart by muchsmaller distances—on the scale of the wavelength, of light signals 112.In fact, typically, surface waveguides 304 are arrayed with a pitch thatis within the range of approximately λ/3 to approximately 2λ. Inembodiments, p1 is within the range of approximately 600 nm toapproximately 2.5 microns. As a result, the scattering process is moreefficient (by 3 orders of magnitude or more), thereby reducing the RFpower required to effect beam steering, as well as the length of thesurface waveguides required.

Furthermore, in contrast to acousto-optic deflection systems based onlow-confinement waveguides, the length of surface waveguides 304 can beshorter—typically within the range of approximately 100 microns toapproximately 1 cm. It should be noted that shorter length can becritical for integrated-optics-based optical waveguides due to the factthat it is difficult to maintain optical coherence over more than a fewmillimeters due to imperfections in the surface waveguides, whichnormally arise during fabrication. It also enables faster beam steering.In some embodiments, the effect of waveguide non-uniformity is mitigatedby fabrication through ultraviolet (UV) lithography on large wafers.

In some embodiments, at least one of surface waveguides 304 is opticallycoupled with a photonic lightwave circuit that includes one or morephotonic elements such as lasers, phase shifters, detectors, gainelements, modulators, diffraction elements, Bragg mirrors, splitters,combiners, and the like.

At operation 403, acoustic transducer 106 is formed on substrate 302such that it is operatively coupled with acousto-optic antenna array102.

In the depicted example, acoustic transducer 106 is an interdigitatedtransducer (IDT 306) that comprises piezoelectric slab 310 andinterdigitated electrodes 312. Piezoelectric slab 310 is a layer ofpiezoelectric material (e.g., aluminum nitride, lithium niobate, leadzirconium titanate (PZT), indium phosphide, gallium arsenide and itscompounds, etc.), which is formed on the top surface of active layer320. In some embodiments, acoustic transducer 106 comprises a differentmechanical transducer, such as a chirped IDT, electro-static actuation,electro-thermal actuation, thermal actuation, magneto-strictiveactuation, optical excitation, etc. In some embodiments, acoustictransducer 106 includes a piezo-optic material, such as lithium niobate(LiNbO₃), and the like.

Electrodes 312 are interdigitated electrically conductive traces formedon the top surface of piezoelectric slab 310. When a periodic drivesignal, such as a sinusoidally varying voltage, is applied to theinterdigitated electrodes, IDT 306 generates acoustic energy 114 whichpropagates into surface waveguides 304 via mechanical phase shifters 308as mechanical waves 326. Each of mechanical phase shifters 308 islocated in the intervening portion of active layer 320 between acoustictransducer 106 and a different surface waveguides 304.

It should be noted that, in the depicted example, mechanical waves 326and light signals 112 are counter propagating in the surface waveguides.In some embodiments, the mechanical waves and light signals propagate inthe same direction in the surface waveguides.

At operation 404, phase controller 108 is formed on substrate 302.

Phase controller 108 comprises a plurality of mechanical phase shifters308, each of which is formed on the top surface of active layer 320.Each mechanical phase shifters 308 is an element that is operative forchanging the speed at which acoustic energy propagates between acoustictransducer 106 and a different one of surface waveguides 304 ofacousto-optic antenna array 102. As a result, each mechanical phaseshifter 308 is operative for controlling the phase of the acoustic wavereceived by the surface waveguide 304 with which it is operativelycoupled.

In the depicted example, each of mechanical phase shifters 308 is athermal element that tunes the elasticity of the region of active layer320 between acoustic transducer 106 and its respective surface waveguide304. In some embodiments, at least one of mechanical phase shifters 308controls the density of the material of active layer 320 in at least aportion of the region between acoustic transducer 106 and each ofsurface waveguides 304. In some embodiments, beam steering in the y-zplane is effected by controlling the phase of light signals 112 usingoptical phase shifters.

Mechanical phase shifters 308 are collectively operative for controllingthe phase relationship of the light scattered out of the surfacewaveguides, which dictates the transverse angle, ϕ, of output beam 116in the y-z plane.

FIG. 5 depicts a schematic of a cross-section view of acousto-opticantenna array 102. The view depicted in FIG. 5 is taken through line b-bas shown in FIG. 3A. The phases, ψ1 through ψ5, of the mechanical waves326-1 through 326-5 in surface waveguides 304-1 through 304-5,respectively, determine the angle, ϕ, in the y-z plane at which outputbeam 116 propagates.

Although the illustrative embodiment includes an acoustic transducer, aphase controller, and an acousto-optic antenna array that aremonolithically integrated on the same substrate (i.e., formed on thesame substrate), it will be clear to one skilled in the art, afterreading this Specification, how to specify, make, and use alternativeembodiments in accordance with the present disclosure in which at leastone of acoustic transducer 106 and phase controller 108 is mechanicallyaffixed to the substrate via hybrid-integration techniques such it isoperatively coupled with acousto-optic antenna array 102. In someembodiments, at least one of acousto-optic antenna array 102, acoustictransducer 106, and phase controller 108 resides on a differentsubstrate and is operatively coupled with the rest of system 100 via anintermediate medium.

At operation 405, light signals 112 are injected into surface waveguides304 by optically coupling light source 104 and acousto-optic antennaarray 102.

FIG. 6 depicts operations of a method for steering an optical beam intwo dimensions in accordance with the illustrative embodiment. Method600 begins with operation 601, wherein light signals 112 are launchedinto surface waveguides 304 in response to a drive signal fromprocessing circuit 110.

While not required, light source 104 is typically optically coupled withacousto-optic antenna array 102 such that light signals 112 arecollectively in phase as they propagate through the surface waveguidesof the antenna array when system 100 is in its quiescent state (i.e.,when no mechanical energy is generated by acoustic transducer 106 and nophase change is proactively imparted on any of the light signals).

At operation 602, processing circuit 110 energizes acoustic transducer106 to generate acoustic energy 114 that couples into each of surfacewaveguides 304 as a different one of mechanical waves 326, giving riseto free-space scatter energy that defines output beam 116.

At operation 603, processing circuit 110 directs phase controller 108 tosteer output beam 116 in the x-z plane by controlling the frequencies ofmechanical waves 326, which determines longitudinal angle θ.

At operation 604 processing circuit 110 directs phase controller 108 tocontrol the relative phases, ψ1 through ψ5, of the mechanical waves326-1 through 326-5 coupled with surface waveguides 304-1 through 304-5to control transverse angle ϕ.

FIG. 7 depicts a beam-steering system in accordance with an alternativeembodiment in accordance with the present disclosure. System 700 isanalogous to system 100 described above; however, system 700 includes ahigh-index-contrast slab waveguide instead of an array of individuallydefined channel-type surface waveguides.

Slab waveguide 702 is a high-confinement slab waveguide that stronglyconfines the light signal provided by light source 104 in thez-direction, thereby resulting in guided wave 704. In the depictedexample, slab waveguide 702 is a layer of single-crystal silicon havinga thickness of approximately 300 nm; however, other dimensions and/ormaterials can be used for slab waveguide 702 without departing from thescope of the present disclosure.

In operation, slab waveguide 702 receives light signal 704 from lightsource 104. Mechanical phase shifters 308 function as a “phased-array”that provides composite mechanical wave 706, which is steered withinslab waveguide 702 by controlling the phase shifts imparted at themechanical phase shifters.

When composite mechanical wave 706 and guided wave 704 interact, lightfrom the guided wave is scattered into free space as output beam 708,whose transverse angle, ϕ, is based on the angle of incidence betweenthe composite mechanical wave and the guided wave.

In some embodiments, system 700 includes a plurality of optical phaseshifters that collectively control the direction of propagation of lightsignal 704 within slab waveguide 702. In such embodiments, the directionof propagation of output beam 708. In some of these embodiments,mechanical phase shifters 308 are not included and the direction ofpropagation of output beam 708 is controlled by controlling thedirection of propagation of light signal 704 and the relativefrequencies of mechanical wave 326 and light signal 704.

FIG. 8 depicts a beam-steering system in accordance with anotheralternative embodiment in accordance with the present disclosure. System800 is analogous to system 700 described above; however, system 800includes a phase controller that includes a plurality of optical phaseshifters configured to control the phases of a plurality of lightportions of light signal 704 as the light portions propagate throughslab waveguide 702.

Each of optical phase shifters 802 is a conventionalintegrated-optics-based phase shifter suitable for controlling the phaseof a portion of light signal 704 as it propagates through the region ofslab waveguide 702 with which it is operatively coupled. In the depictedexample, each of phase shifters 802 is a thermal phase shifter. Otheroptical phase shifters suitable for use in embodiments in accordancewith the present disclosure include stress-based phase shifters, surfaceacoustic wave-based (SAW-based) phase shifters, and the like.

The light portion that propagates into slab waveguide 702 from each ofoptical phase shifters 802 has a phase that is controlled by that phaseshifter. Collectively, the plurality of light portions gives rise tocomposite optical signal 804 whose propagation direction within the slabwaveguide is based on their relative phases.

In analogous fashion to the operation of system 700, the transverseangle of output beam 806 is based on the angle of incidence betweenmechanical wave 326 and composite guided wave 804, which is controlledby optical phase shifters 802.

In some embodiments, both optical phase shifters 802 and mechanicalphase shifters 308 are included.

It is to be understood that the disclosure teaches just some examples ofthe illustrative embodiment and that many variations of the embodimentsdescribed herein can easily be devised by those skilled in the art afterreading this disclosure and that the scope of the present invention isto be determined by the following claims.

What is claimed is:
 1. An optical-beam steering system comprising: asubstrate; a planar lightwave circuit (PLC) comprising at least onesurface waveguide that is formed on the substrate such that the PLC andsubstrate are monolithically integrated, each of the at least onesurface waveguide conveying at least a portion of a light signal, andthe at least one surface waveguide defines a mechanically active regionthat is movable relative to the substrate; an acoustic transducer thatis configured to induce at least one mechanical wave in the at least onesurface waveguide; and a phase controller comprising a plurality ofphase shifters that includes a first phase shifter configured to controlat least one of (1) the phase of a first mechanical wave of the at leastone mechanical wave and (2) the phase of a first light portion of thelight signal in a first surface waveguide of the at least one surfacewaveguide.
 2. The optical-beam steering system of claim 1: wherein theat least one surface waveguide includes a plurality of surfacewaveguides, each surface waveguide of the plurality thereof beingmovable relative to the substrate, and each surface waveguide of theplurality thereof conveying a different light portion of the lightsignal; wherein the at least one mechanical wave includes a firstplurality of mechanical waves, each of which is coupled into a differentsurface waveguide of the plurality thereof; wherein each phase shifterof the plurality thereof is configured to control the relative phase ofeach mechanical wave of the first plurality thereof and the lightportion conveyed by its respective surface waveguide.
 3. Theoptical-beam steering system of claim 2 wherein the first phase shifteris operative for controlling a physical property of a first materialregion located between the acoustic transducer and the first surfacewaveguide.
 4. The optical-beam steering system of claim 3 wherein thephysical property is selected from the group consisting of elasticityand density.
 5. The optical-beam steering system of claim 2 wherein eachof the plurality of phase shifters is a mechanical phase shifter that isoperative for controlling a different material region of a pluralitythereof, wherein each material region is located between the acoustictransducer and a different surface waveguide of the plurality thereof.6. The optical-beam steering system of claim 2 wherein each of theplurality of phase shifters is an optical phase shifter that isconfigured to impart a phase shift on a different portion of the lightsignal.
 7. The optical-beam steering system of claim 2 wherein the lightsignal is characterized by a wavelength, and wherein each of theplurality of surface waveguides includes a first portion, and whereinthe plurality of first portions is arranged in a linear array having auniform pitch that is within the range of approximately λ/3 toapproximately 2λ.
 8. The optical-beam steering system of claim 1 whereinthe PLC defines a first plane, and wherein the plurality of surfacewaveguides and the acoustic transducer are configured such that at leastone light portion of the plurality thereof and at least one mechanicalwave of the first plurality thereof interact to scatter optical energyof the at least one light portion out of the first plane as at least oneoutput beam.
 9. The optical-beam steering system of claim 1 furthercomprising a plurality of supports that support the mechanically activeregion above the substrate, each support being located between a pair ofadjacent surface waveguides of the plurality thereof, and each supportbeing configured to mitigate mechanical coupling between its respectivepair of adjacent surface waveguides.
 10. The optical-beam steeringsystem of claim 1 wherein the at least one surface waveguide includes aslab waveguide, and wherein the at least one mechanical wave includes afirst plurality of mechanical waves that propagate in the slabwaveguide, and further wherein the phase controller is configured tocontrol the relative phases of the first plurality of mechanical waves.11. The optical-beam steering system of claim 1 wherein the at least onesurface waveguide includes a slab waveguide, and wherein the lightsignal includes a plurality of light portions, and further wherein thephase controller is configured to control the relative phases of theplurality of light portions.
 12. A method for steering an optical beam,the method comprising: conveying a light signal having wavelength λ inat least one surface waveguide of a planar-lightwave circuit (PLC) thatis formed on a substrate such that the at least one surface waveguideand the substrate are monolithically integrated, wherein the at leastone surface waveguide defines a mechanically active region that ismovable relative to the substrate; coupling a plurality of mechanicalwaves into the at least one surface waveguide such that interactionbetween the plurality of mechanical waves and the at least one lightsignal gives rise to emission of an optical beam from the plurality ofsurface waveguides, wherein the mechanical waves are characterized byfirst frequency, ω; and controlling the first frequency, ω.
 13. Themethod of claim 12 wherein the at least one surface waveguide includes aplurality of surface waveguides that are movable relative to thesubstrate, and wherein the light signal includes a plurality of lightportions, each of which propagates in a different surface waveguide ofthe plurality thereof, and further wherein a different mechanical waveof the plurality thereof is coupled into each of the surface waveguidesof the plurality thereof.
 14. The method of claim 13 further comprisingcontrolling a first relative phase of a first light portion of theplurality thereof and a first mechanical wave of the plurality thereofin a first surface waveguide of the plurality thereof, wherein the firstrelative phase is at least partially controlled by controlling the phaseof the first mechanical wave.
 15. The method of claim 14 wherein thephase of the first mechanical wave is controlled by controlling aphysical property of a first material region located between an acoustictransducer and the first waveguide, the physical property being selectedfrom the group consisting of elasticity and density.
 16. The method ofclaim 13 further comprising controlling a first relative phase of afirst light portion of the plurality thereof and a first mechanical waveof the plurality thereof in a first surface waveguide of the pluralitythereof, wherein the first relative phase is at least partiallycontrolled by controlling the phase of the first light portion.
 17. Themethod of claim 13 further comprising controlling the relative phases ofthe plurality of mechanical waves.
 18. The method of claim 13 furthercomprising controlling the relative phases of the plurality of lightportions.
 19. The method of claim 13 wherein each of the plurality ofsurface waveguides includes a first portion, and wherein the pluralityof first portions is arranged in a linear array having a uniform pitchthat is within the range of approximately λ/3 to approximately 2λ. 20.The method of claim 12 wherein the PLC defines a first plane, andwherein the method further includes: interacting at least one lightportion of the plurality thereof and at least one mechanical wave of thefirst plurality thereof to scatter optical energy of the at least onelight portion out of the first plane to define the optical beam.
 21. Themethod of claim 12 wherein the at least one surface waveguide includes aslab waveguide, and wherein the plurality of mechanical waves is coupledinto the slab waveguide.
 22. The method of claim 21 wherein the at leastone surface waveguide includes a slab waveguide, and wherein theplurality of mechanical waves is coupled into the slab waveguide as acomposite wave.
 23. The method of claim 22 further comprisingcontrolling the phases of the plurality of mechanical waves, wherein thepropagation direction of the composite wave in the slab waveguide isbased on the phases of the plurality of mechanical waves.
 24. The methodof claim 22 further comprising controlling the phases of a plurality oflight portions included in the light signal, wherein the propagationdirection of the light signal in the slab waveguide is based on thephases of the plurality of light portions.